Fundamentals on Bionanotechnologies - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 29

showing a staircase shape, are not retrieved in the bulk materials from which they
are fabricated.
A nanowire confines the carriers in two dimensions. The NW has a diameter
of a few nm and lengths of 100-300 nm, but longer nanowires were also fabri-
cated. Nanowires can be obtained through many methods, for example, bottom-up
approaches ( Bowler 2004 ). Another method to fabricate nanowires is based on
Si(001), which is more reactive if not hydrogenated. Therefore, hydrogen can be
used as a nanoscale mask, and other materials adsorbed on the nonhydrogenated
regions can generate a nanowire. For example, (low-quality) Fe, Al, Co, Ga, or Ag
nanowires can be obtained in the depassivated regions. There are several methods to
fabricate self-assembled nanowires, such as methods based on specific chemical
reactions and surface forces. For instance, rare-earth nanowires are fabricated
by depositing the rare-earths on a Si(001) substrate and subsequent annealing.
Similarly, Bi nanowires with a length of 200-600 nm and a width of 1.5 nm appear
spontaneously when a Si(001) substrate covered with Bi is annealed around 600 ı C.
Template synthesis is, however, the most common method to fabricate nanowires.
The template is, in fact, a prescribed pattern, for example, hexagonal, of nanosized
pores or voids in a host material. Nanowires form when these pores are filled with
the constituent material, termed source material. There are few reviews on nanowire
growth such as Wang and Zhang ( 2008 )or Bandaru and Pichanusakorn ( 2010 ).
Figure 1.26 illustrates the most common template for nanowire growth, which
is anodic alumina. The alumina template results from anodizing Al thin films in
specific acids. If an electrical current flows between the Al film and the cathode
during anodization, an alumina film (membrane) displaying a regular hexagonal
array of parallel and nearly identical cylindrical pores is obtained as the result of the
etching process, as illustrated in Fig. 1.26 . These nanopores have diameters within
the 20-200-nm range, pitches of 50-400 nm, and a density that varies between 10 9
and 10 11 cm 3 as a function of the etching conditions. The anodization process
produces regular pore geometries. Nanochannels in glasses and etched polymers
are also widely used to fabricate nanowires.
Precise templates can be created also by chemical etching of particle tracks
resulted from ion bombardment. Nearly all types of nanowire can be fabricated
using template techniques followed by a deposition method, and a subsequent
extraction procedure of the source material.

anodized alumina

source
material

aluminium

Fig. 1.26 Nanowire growth
in an alumina template